Wah How to use "So do I / Neither do I"

You can use So do I or Neither do I when you want to say something that is the same as others.

Positive sentence :  So + auxiliary verb + pronoun  :So do I, So am I, So have I...
Negative sentence : Neither + auxiliary verb + pronoun  : Neither do I, Neither am I, Neither have I...

For examples:

A: I am so excited.     
B: So am I.

A: I am not hungry.  
B: Neither am I.

A: John likes chocolate. 
B: So does Helen.  

A: Helen does not want to go out tonight.   
B: Neither dose John.

A: I have never been to Tokyo.
B: Neither have I. 

A: I can come the party tonight.
B: So can I.